Control Strategy Research on Avoid and Escape the Singular Area of the 3PRR Parallel Platform Based on SEM Environment

Micro-nano operating system is an important part in the field of precision operation and machining. The sample stage of the micro-nano operating system is used to place and move samples at large-scale stroke; therefore the sample stage needs to possess the features of large stroke and high precision positioning. Macro-micro combination method can make up the stroke deficiency of the precision positioning platform, whereas parallel mechanism can realize high precision positioning. Hence, under the environment of the scanning electron microscopy (SEM), the 3PRR parallel positioning stage, which has 3 degrees of freedom and whose chains each consist of a prismatic joint and two revolute joints, is adopted to constitute the macro-movement part of the macro-micro combination precision positioning platform. In order to achieve the large stroke and high precision positioning, the workspace and singularity distribution of the 3PRR require to be analyzed. The Jacobi matrix is obtained by calculating the kinematics equation. And after the analysis of the characteristics of the Jacobi matrix, the nonsingular workspace distribution of the 3PRR stage is acquired. Then, based on the characteristics of the Jacobi matrix and the singular workspace distribution, two control strategies are presented for avoiding singular area, and one control strategy for escaping the singular position. The corresponding theory of the strategies is provided. The simulation and analytic results show that these strategies can ensure the precision positioning and controllability of the mechanism.